Determining the Structure of Antimony Oxychloride Glass
Robin Orman (Warwick), Diane Holland (Warwick), Alex Hannon (ISIS)
Introduction
Abstract
Sb2O3-based glasses are of interest due to the lone
pair of electrons on the antimony atom and the nonlinear optical properties that may arise from it.
Crystalline onoratoite (Sb8O11Cl2) has been
prepared and a portion vitrified by splat-quenching
of the melt. The two samples have been compared
with an earlier Sb2O3-SbCl3 glass using Raman
spectroscopy and examined using neutron
diffraction. The Raman data confirms that the new
antimony oxychloride glass is essentially identical to
the older sample and has a structure based on that
of crystalline onoratoite. The crystal neutron data
suggests that a recent, complex structural model is
more accurate than an older one with oxygen
disorder. The glass neutron data supports the
Raman data in showing that the structure is broadly
similar to the crystal. Rietveld refinement and RMC
techniques will be used to analyse the data further.
Sb2O3–SbCl3 glass
Onoratoite glass
Onoratoite (Sb8O11Cl2)
A chlorine-stabilised Sb2O3 glass was
prepared by Johnson et al. [1] from the melting of an
equimolar mixture of Sb2O3 and SbCl3, and this
system has been the subject of a more detailed
study by Orman [2].
In Orman’s work, differential thermal
analysis showed that the glass exhibited similar
thermal events to those of the crystalline antimony
oxychloride Sb8O11Cl2 (onoratoite) and Raman
spectroscopy showed the structure to be similar.
Energy dispersive X-ray analysis also estimated the
chlorine content to be ~8.5 at.%, similar to that
predicted for onoratoite (9.5 at.%).
Menchetti’s model
After a single-crystal XRD
experiment, Menchetti et al.
[4] proposed a model based
on a simple antimony-oxygen
‘ladder’ structure. The chains
link to form self-contained
‘tubes’ of atoms.
Onoratoite: crystal and glass
100
200
300
400
Oxygen
Chlorine
600
700
800
900
-1
Shift (cm )
Figure 1 – Spectra obtained at room
temperature on a Renishaw Invia
Raman spectrometer using a 20mW
laser source of wavelength 514nm.
Raman
spectroscopy
shows the new glass to be almost
identical to the earlier Sb2O3SbCl3 glass, and similar to the
crystalline onoratoite.
Neutron diffraction of
both forms of onoratoite was
carried out on the GEM
diffractometer at the ISIS neutron
source, UK.
Mayerová’s model
Antimony
4 of the 6 oxygen
positions are partially
occupied, resulting in 3/8
of the antimony atoms
being 3-coordinated, the
rest 4-coordinated.
500
Crystalline
onoratoite
was
prepared according to the method
of Matsuzaki et al. [3], melted in a
lidded alumina crucible and splatquenched to form a glass.
More recently, Mayerová et al.
[5] conducted a new X-ray
diffraction study and developed
their own model – a more
complex version of Menchetti’s.
5/16 of the Sb are 3coordinated and Sb–Cl
distances are 2.95-3.20Å –
shorter
than
in
the
Menchetti study but still
unusually long.
This new model has two types
of ladder chain, each one
interrupted by [SbO3] groups.
One oxygen site also forms
links between the tubes within
each layer.
The tubes form alternating
layers with the chlorine
atoms,
giving
rise
to
unusually
wide
Sb–Cl
separations (3.2-3.8 Å).
Which model is more accurate? (1)
Which model is more accurate? (2)
Glass and crystal comparison
Simulated correlation functions for the two models of
crystalline onoratoite (generated using the XTAL program
[6]) have been compared with the neutron data.
A comparison of the data with Mayerová’s model is more
favourable (Fig. 3).
Both the Sb–O and the O–O
contributions reproduce the observed data with a high
degree of accuracy; the small discrepancy in the position of
the first O–O peak may be attributable to a less-than-perfect
merging of the information from different detector banks, or
due to small inaccuracies in oxygen positions in the model.
The data from the neutron diffraction appears to support the
Raman data in that the onoratoite glass structure seems to
exhibit similar atomic correlations to the crystal. The
absence of intensity at ~2.1 Å in the glass probably
indicates shorter and more uniform Sb–O bonds than in the
crystal, perhaps due to the relaxation of the requirement for
long-range atomic ordering.
It is noticeable from the previous work [5] that the
Bond Valence Sum (BVS) parameters for the chlorine
atoms in this structural model are of the order of 0.2-0.25
(i.e. significantly lower than the expected value of 1.0). The
antimony and oxygen atoms have, approximately, their
expected BVS values (3.0 and 2.0, respectively).
We plan to use Rietveld refinement to improve the
structural model of crystalline onoratoite – previous studies
have used X-ray diffraction, so the neutron data should
provide better information on the lighter atoms – and
Reverse Monte Carlo modelling to determine the glass
structure.
Figure 3 – Simulated correlation functions for
Mayerová’s model compared with the neutron data
(partial correlation functions not shown do not influence
the total below 2.9 Å). Simulated thermal parameters
for the Sb–O and O–O correlations have been adjusted
to give the best fit with the data.
Figure 4 – A comparison of the onoratoite glass
and crystal neutron diffraction data.
Menchetti’s model (Fig. 2) shows a noticeable
deviation from the observed data. The model predicts half
of the Sb–O separations to be 2.2 Å (the links along the
‘ladder’) whilst the perpendicular Sb–O ‘rungs’ of the
ladder are 1.9-2.1 Å. This leads to roughly equivalent
peaks in the correlation function at 2.0 Å and 2.2 Å –
however, the data shows that a large majority of the Sb–O
separations are of the shorter variety, with only a small
proportion of longer bonds. The shortest O–O distance
predicted by Menchetti, arising from the oxygen atoms in
the plane of the ladder, is also noticeably shorter than that
actually observed.
Figure 2 – Simulated partial correlation functions for
Menchetti’s model compared with the neutron data
(partial correlation functions not shown do not
influence the total below 2.9 Å). Simulated thermal
parameters for the Sb–O and O–O correlations have
been adjusted to give the best fit with the data.
References
1. J. A. Johnson, D. Holland, J. Bland, C. E. Johnson, and M. F. Thomas, J. Phys.: Condens. Matter 15 (2003) 755-764.
2. R. G. Orman, MSc Thesis, University of Warwick, 2005.
Conclusions
From the analysis of the neutron diffraction data to date, it
appears that Mayerová’s model of the onoratoite crystal
structure is more accurate than Menchetti’s. This will prove
useful in determining the structure of the glass since there is
evidence from Raman spectroscopy that the glass structure
is strongly related to that of the crystal.
3. R. Matsuzaki, A. Sofue, and Y. Saeki, Chem. Lett. 12 (1973) 1311-1314.
4. S. Menchetti, C. Sabelli, and R. Trosti-Ferroni, Acta Crystallogr. C 40 (1984) 1506-1510.
5. Z. Mayerová, M. Johnsson, and S. Lidin, Solid State Sci. 8 (2006) 849-854.
6. A.C. Hannon, XTAL: A program for calculating interatomic distances and coordination numbers for model
structures, Rutherford Appleton Laboratory Report RAL-93-063, 1993.
Acknowledgements
RGO would like to thank EPSRC for a studentship.